A material used for manufacturing a liquid crystalline polymer film having a birefringence property has a molecular structure similar to that of a typical liquid crystal before a photocuring reaction is induced by UV irradiation. Each of photocurable liquid crystalline monomer molecules has a rod-shaped molecular structure configured with a hard molecule to induce a liquid crystal phase and one or more photoreactive groups attached to ends thereof, and a photo-initiator is mixed into the material. The aforementioned photocurable liquid crystalline monomer constitutes a liquid crystalline polymer film by polymerization between constituents generated by the UV irradiation.
Since the liquid crystalline polymer film using the photocurable liquid crystalline monomer is formed in a monomer state by a film forming process at the time of inducing alignment according to a condition of an interface, there is an advantage in that relatively good alignment characteristics can be obtained in comparison with the case where the film forming process is performed by using a liquid crystalline polymer material which is polymerized in advance at the time of the film forming process. Therefore, in recent years, the liquid crystalline polymer film is widely used in the fields of display and the fields of optical elements requiring uniform optical anisotropy or patterned optical anisotropy.
FIG. 1A is a conceptual diagram illustrating a state of a photocurable liquid crystalline monomer which is aligned before photocuring, and FIG. 1B is a conceptual diagram illustrating a state of a liquid crystalline polymer film which is aligned and polymerized in a thin-film shape by photocuring reaction.
As described above, due to the existence of the liquid crystalline monomer at the time of inducing the alignment, a film can be easily formed by a coating process such as bar coating or spin coating, and the same condition of alignment film as the case where liquid crystal is aligned is formed in the lower portion, so that the film aligned according to a condition of an interface can be formed as illustrated in FIG. 1A. As illustrated in FIG. 1B, after the alignment is induced, a remaining organic solvent is removed by thermal treatment, and photopolymerization between monomers is induced by UV irradiation, so that a stabilized liquid crystalline polymer film with the alignment retained is formed.
In the formation of the liquid crystalline polymer film, before the photocuring reaction is induced by the UV irradiation, similarly to a typical liquid crystal, in the photocurable liquid crystalline monomer, a degree of alignment in the alignment direction with respect to the alignment axis varies with a temperature condition. A quantitative physical value of the degree of alignment is referred to as an order parameter S, which is expressed by Mathematical Formula 1.
                              S          ⁡                      (            T            )                          =                  〈                                                    3                ⁢                                  cos                  2                                ⁢                θ                            -              1                        2                    〉                                    [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            
Herein, the order parameter S is a function varying with temperature and indicates an angle between an average molecular axis direction of each liquid crystal molecule or photocurable liquid crystalline monomer molecule in an infinitesimal volume and an axis of each molecule. FIGS. 2A and 2B are a graph illustrating a change in the order parameter S of the liquid crystalline monomer according to temperature and a conceptual diagram for explaining θ. As illustrated in FIGS. 2A and 2B, even in the case of a liquid crystal or a liquid crystalline monomer having a liquid crystal phase at a room temperature, as the temperature is increased, the order parameter S is gradually decreased, and at TNI (nematic-isotropic phase transition temperature), the liquid crystal or the liquid crystalline monomer is phase-transitioned into an isotropic liquid. If S=0, the liquid crystal or the liquid crystalline monomer becomes an isotropic liquid, and the optical anisotropy (birefringence) disappears. As the order parameter is close to S=1, the liquid crystal molecules are well aligned along the alignment direction. In general, the order parameter of the liquid crystal or the liquid crystalline monomer is S<1 at the room temperature, and the case of S=1 indicates a monocrystalline state.
FIG. 3 is a graph illustrating a change of an ordinary refractive index no and a change of an extraordinary refractive index ne of the liquid crystalline monomer according to temperature. As illustrated in FIG. 3, although the ordinary refractive index no and the extraordinary refractive index ne are given in the molecular level of the liquid crystalline monomer, since the refractive index macroscopically measured on the film is obtained with respect to each optical axis as an average value due to the change of the order parameter according to temperature, as the temperature is increased, the extraordinary refractive index ne is gradually decreased, and the ordinary refractive index no is gradually increased. At the temperature equal to or higher than TNI ni (nematic-isotropic phase transition temperature), the liquid crystalline monomer is phase-transitioned into an isotropic liquid, and the optical anisotropy disappears, so that the liquid crystalline monomer has a single refractive index ni. Herein,
      n    i    =                              n          e                +                  2          ⁢                      n            o                              3        .  Namely, the ordinary refractive index and the extraordinary refractive index of the liquid crystalline monomer are a function of temperature.
On the other hand, in order to obtain the photocurable liquid crystalline polymer film aligned as illustrated in FIG. 1B, the lower substrate where coating is to be performed needs to be applied with an alignment process. In general, this alignment method is achieved by forming the film by coating a PVA-based or PI-based alignment film, and after that, defining the alignment direction of the liquid crystalline monomer formed on the film by a rubbing process. Alternatively, by irradiating an alignment film having a photo-alignment characteristic with polarized UV, alignment of the photocurable liquid crystalline monomer formed on the alignment film can be induced. At this time, a multi-aligned liquid crystalline polymer film can be formed by forming the alignment of the lower substrate through spatial multiple division. As an example of the multi-aligned liquid crystalline polymer film, there is a patterned retarder implementing a 3D image in a polarized glasses method.
However, there is a problem in that, as a thickness of a to-be-formed liquid crystalline polymer film is increased, a distance to the lower alignment film is increased, so that the alignment effect due to the lower alignment film is gradually decreased. In addition, there is a problem in that, as it goes to an upper portion of the liquid crystalline polymer film, the alignment state becomes poor. As another example of the 3D display implementing method, there is a polarization-dependent lens array using a liquid crystalline polymer required by a 2D/3D switching display. In order to manufacture a lens array having a short focal length by using a liquid crystalline polymer, each lens is formed to have sufficiently different phase retardation, and thus, a liquid crystalline polymer film is necessarily formed to have a large thickness.
In this case, in order to solve the above-described problems, since sufficient alignment effects cannot be obtained by only the bottom-up alignment by the lower alignment film, alignment needs to be induced from an upper portion by a top-down alignment method in the formation of the liquid crystalline polymer film. According to the top-down alignment method, alignment can be additionally induced by forming an interface having a groove structure in the upper portion thereof. The direction of the above-described top-down alignment can be induced along the groove direction.
FIGS. 4A and 4B are polarizing microscope pictures illustrating alignment states of thick photocurable liquid crystalline polymer film formed with a thickness of 20 μm, FIG. 4A is a picture with respect to the polarization axis of a polarizing microscope, and FIG. 4B is a picture in the case of a sample is disposed at 45 degrees with respect to the polarization axis of the polarizing microscope. In FIGS. 4A and 4B, in order to test the influence of the upper alignment in the thick liquid crystalline polymer film, a portion of the sample is aligned with only the lower alignment condition, and the remaining portion is aligned with the lower alignment condition and the upper alignment condition. In each of the pictures of FIGS. 4A and 4B, the left image is an alignment picture of a region formed with only the lower alignment condition by a rubbed PI film, and the right image is an alignment picture of a region formed by alignment induced with the lower alignment condition by the rubbed PI film and the upper alignment condition by the groove structure. As illustrated in the polarizing microscope pictures obtained by observation while changing the condition of an angle between a transmission axis of a polarizer and the alignment axis, it can be seen that, in the case of the thick liquid crystalline polymer film, uniform alignment cannot be induced by only the lower alignment film, and however, in the case of utilizing the bottom-up and top-down alignment effect simultaneously, uniform alignment state can be obtained.
However, as described above, in the inducement of the upper alignment, a groove pattern is performed in the upper portion of the formed liquid crystalline polymer film by the groove structure. The groove pattern formed on the upper surface of the liquid crystalline polymer film causes a problem in that undesired optical diffraction phenomenon occurs and, thus, optical noise is generated.
For example, in the case of forming a polarization-dependent liquid crystalline polymer lens array, besides the condensing effect of the lens, diffraction light is generated, and thus, optical noise is generated. The optical noise causes a noise image in display of a 3D image or a 2D image. Therefore, an additional technique for removing the optical noise caused by the diffraction light is needed.